1. Field of the Invention
This invention relates to a laser output control for a optical disk apparatus of a CAV recording system designed to record information by optical laser power in an optical disk such as CD-R, CD-RW, CD-WO, MD, DVD, DRAW or the like.
2. Background Art
With regard to a DRAW optical disk or a rewritable optical disk, for example, a dye-containing recording material or the like is coated, and a record pit is formed thereon by irradiating the recording material with a laser light beam. To stably form the record pit on the optical disk under a constant condition, a laser diode (LD) driving current must be controlled such that always the constant laser power can be obtained. As a recording signal waveform of the laser light, for example as shown in FIG. 4, a basic pattern is a rectangular waveform raised from a bottom level BP (=read level RP) to a write level WP for forming a record pit. Variation of the read level and the bottom level adversely affect focusing and tracking servo gains. Also, variation of the write level affects the formation quality of the record pit. Thus, to realize a stable recording/reproducing operation, LD driving control must be performed such that the read level RP, the bottom level BP and the write level WP can always maintain respective target levels.
FIG. 5 is a block diagram showing the configuration of a laser output control unit for a conventional optical disk recording/reproducing apparatus designed to perform such LD driving control. A laser light from a laser diode (LD) 2 driven by a laser driving device 1 is irradiated to an optical disk for reading information written in the optical disk, and for writing information in the optical disk. The target value of the output power of the laser light in this case is equal to the read level RP of FIG. 4 in the reading mode, and equal to the bottom level BP or the write level WP of FIG. 4 in the write mode. A photodiode (PD) 3 receives a part of a reflected light from the optical disk or a part of the laser light irradiated to the optical disk, and converts the received light into an electric signal. A light signal outputted from this PD 3 is converted from a current signal into a voltage signal by an I/V converter 4, and is outputted as a laser output detecting signal I/VOUT. The laser output I/VOUT is supplied to a read/bottom power detecting circuit 5 and a write power detecting circuit 6. The read/bottom power detecting circuit 5 detects a read power RP by sampling and holding the laser output detecting signal I/VOUT at a predetermined time interval on the reading mode, and detects a bottom power BP by sampling and holding the bottom part of the rectangular wave of the laser output detecting signal I/VOUT on the writing mode. The write power detecting circuit 6 detects a write power RP by sampling and holding the peak part of the rectangular wave of the laser output detecting signal I/VOUT on the writing mode. Sampling and holding timings at the read/bottom power detecting circuit 5 and the write power detecting circuit 6 are controlled by a laser power switching control unit 7 in accordance with an eight to fourteen modulation (EFM) signal representing a recording signal. Regarding a read/bottom power detecting signal R/BPD outputted from the read/bottom power detecting circuit 5, a read/bottom power target value R/BPRef is subtracted by a subtracter 8 to generate a read/bottom power control signal R/BPC, and this signal is supplied to A terminal side of a switching device 10. From a write power detecting signal WPD outputted from the write power detecting circuit 6, a write power target value WPRef is subtracted by a subtracter 9 to generate a write power control signal WPC, and this signal is supplied to B terminal side of the switching device 10. The switching device 10 is subjected to switching control by the laser power switching control unit 7 in accordance with the EFM signal, and either of the control signals R/BPC and WPC is selectively supplied to the laser driving device 1.
FIG. 6 is a time chart showing the operation of the conventional laser output control apparatus constructed in the foregoing manner. During the reading mode, the laser output control apparatus controls the laser driving device 1 such that the switching device 10 is fixed to the A terminal side, and the laser power can be set to a constant read level RP, which is lowered for forming no pits on the disk. During the writing mode, the laser output control apparatus controls the laser driving device 1 such that at a pit portion, the switching device 10 is connected to the B terminal side, and a pit can be formed on the disk by raising the laser power to the write level. At a land portion, the switching device 10 is connected to the A terminal side, and a constant bottom level BP lowered for forming no pits on the disk can be set.
In the optical disk apparatus for performing a constant angular velocity (CAV) recording (including partial CAV recording), since a recording operation at a constant recording density is carried out while maintaining the rotational speed of a spindle constant, a recording rate (linear velocity) is increased as a recording position on the optical disk is shifted from the inner periphery to the outer periphery. On the other hand, a bottom level BP in the writing mode is obtained by sampling and holding the bottom part of the rectangular wave of the laser output detecting signal I/VOUT. However, as the recording rate is increased, the response speed of the detection system including the PD3 cannot follow and, as indicated by the dotted-line portion of the enlarged view of the laser output detecting signal I/VOUT in FIG. 6, waveform deformation occurs in the laser output detecting signal I/VOUT. If this dull portion is sampled and held, then the offset occurs between the detecting signal R/BRD of the read/bottom power detecting circuit 5 and an actual bottom level. Such an offset causes the laser driving device 1 to lower a bottom level during the recording. Consequently, a laser driving signal R/BP becomes lower than an optimal value. This offset is also affected by the write power and, if the write power is increased following the increase of the recording rate, the offset is increased more.
If there is a variation in the bottom level due to such a change of the recording rate, then focussing and tracking gains are changed, bringing about the problem of unstable servo control. Further, if the offset is large, laser output may disappear.